Variable: file-coding-system-alist

Alist to decide a coding system to use for a file I/O operation.
The format is ((PATTERN . VAL) ...),
where PATTERN is a regular expression matching a file name,
VAL is a coding system, a cons of coding systems, or a function symbol.
If VAL is a coding system, it is used for both decoding and encoding
the file contents.
If VAL is a cons of coding systems, the car part is used for decoding,
and the cdr part is used for encoding.
If VAL is a function symbol, the function must return a coding system
or a cons of coding systems which are used as above. The function is
called with an argument that is a list of the arguments with which
`find-operation-coding-system' was called. If the function can't decide
a coding system, it can return `undecided' so that the normal
code-detection is performed.

See also the function `find-operation-coding-system' and the variable `auto-coding-alist'.